Water supplies contaminated with arsenic (As) are a major health and environmental concern in the United States and worldwide. Arsenic is a naturally occurring element that is present in rocks or soils. Over time, the arsenic leaches from the rocks or soils into groundwater, surface water, wells, or other sources of drinking water. This arsenic contamination is referred to as indigenous arsenic contamination. Arsenic-contaminated solutions are also produced in a variety of industries, such as mining, agriculture, semiconductor, or petroleum industries. These arsenic-contaminated solutions include process solutions and waste streams. Arsenic is a known carcinogen that has been shown to cause bladder, kidney, liver, and lung cancer. In 1976, the Environmental Protection Agency (“EPA”) established drinking water standards for arsenic at a maximum concentration limit (“MCL”) of 50 μg/L or 50 parts per billion (“ppb”). However, even trace amounts of arsenic have been shown to have health risks, which led the EPA to reduce the MCL to 10 ppb. The reduced MCL will be enforced in 2006, which provides only a few years for development and testing of improved technologies for arsenic removal.
Arsenic is present in nature in valence states or oxidation states of −3, 0, +3, and +5. In water supplies, arsenic contaminants typically exist as As(III) compounds and/or As(V) compounds. The As(III) compounds include As(III) oxyanions or oxyacids, such as H3AsO3 or HAsO2, depending on the pH of the water supply. The As(V) compounds include As(V) oxyanions, such as H2AsO4− or HAsO42−, or oxyacids, such as H3AsO4, depending on the pH of the water supply. Under atmospheric conditions or an oxidizing environment, As(V) compounds are predominantly present in water supplies. As(III) compounds are also known as arsenites, while As(V) compounds are known as arsenates.
Numerous techniques for removing arsenic from water supplies have been proposed and developed. For instance, arsenic removal has utilized anion exchange, cation exchange, polymeric anion exchange, liquid-liquid extraction, activated alumina sorption, coprecipitation, sorption by iron oxide coated sand particles, enhanced coagulation with alum or ferric chloride dosage, ferric chloride coagulation followed by microfiltration, pressurized granulated iron particles, iron oxide doped alginate, manganese dioxide coated sand, polymeric ligand exchange, and zero-valent iron. These techniques primarily rely on ion exchange and Lewis acid-base interactions to remove the arsenic.
In U.S. Pat. No. 5,591,346 to Etzel et al., an iron(III)-complexed cation exchange resin is disclosed for removing arsenic from wastewater or drinking water. The iron(III)-complexed cation exchange resin is formed by loading a strong acid cation exchange resin with iron ions. The cation exchange resin is purchased commercially and then loaded with the iron ions. When the iron(III)-complexed cation exchange resin is contacted with a stream of wastewater or drinking water, the iron ions react with arsenate anions to form an iron arsenate salt complex. The iron arsenate salt complex is immobilized on the cation exchange resin, removing the arsenic from the wastewater or drinking water.
While many techniques for removing arsenic from water supplies are known, conventional ion exchange resins do not provide the specificity to economically remove low concentrations of arsenic. Since many water supplies in the United States, such as groundwater, surface water, or wells, have low concentrations of arsenic, these techniques are not effective to remove the arsenic. In addition, many of these techniques are not selective for arsenic over other ions. To improve the selectively of ion exchange resins for arsenic, granules of metal oxides or metal hydroxides, such as ferric hydroxide, have also been investigated. While these metal oxide or metal hydroxide granules are more selective for arsenic, they have a low porosity and, therefore, have a low capacity for arsenic and poor kinetic properties. To improve the performance of ferric hydroxide, ferric hydroxide has been incorporated into organic polymers. For instance, in “Arsenic removal using a polymeric/inorganic hybrid sorbent” DeMarco et al., Water Research 37 (2003) 164-176, a hydrated iron oxide is dispersed into a polymeric, cation exchange bead. The polymeric, cation exchange beads are commercially available and include a polystyrene matrix having sulfonic acid functional groups. A sorbent is prepared by loading Fe(III) onto the sulfonic acid sites on the cation exchange beads. The Fe(III) is then desorbed and Fe(III) hydroxides are simultaneously precipitated within the cation exchange beads using a strong alkaline solution, encapsulating the hydrated iron oxides within the cation exchange beads. The capacity of the sorbent for arsenic is limited by the total number of sulfonic acid sites on the cation exchange beads. In this sorbent, the hydrated iron oxide is loaded at approximately 0.9-1.2% by mass. In other words, only 9 mg of iron per gram of sorbent is loaded at saturation.
In further attempts to improve techniques for removing arsenic from water supplies, an ion exchange resin for removing trace amounts of ions is disclosed in U.S. Pat. No. 4,576,969 to Echigo et al. The ion exchange resin includes a phenolic resin and a metal hydroxide. The ion exchange resin is formed by incorporating a metal salt or metal oxide into the phenolic resin, which is prepared by precondensating a phenol compound with an aldehyde compound and an acid catalyst. The mixture of the metal salt or metal oxide and the phenolic resin is then subjected to a suspension polycondensation reaction in halogenated solvents. The ion exchange resin is isolated from the reaction mixture and treated with an alkaline agent to produce a spherical ion exchange resin. However, the foregoing process of preparing this ion exchange resin is problematic because it is complicated and the halogenated solvents used in the process are difficult to dispose of.
It is expected that large cities, which typically have centralized water treatment facilities, will be able to comply with the new MCL for arsenic using existing technologies, such as precipitation or coagulation treatment. However, small and mid-sized municipalities and other communities as well as rural areas do not have centralized water treatment facilities. Rather, they typically use point of origin treatment schemes, such as chlorination, at the well head prior to distribution. It is estimated that over four thousand municipal water supplies and tens of thousand of private drinking water supplies, primarily in the western United States, would not meet the new MCL standard of 10 ppb arsenic. Under existing technologies, these small and mid-sized municipalities and private users would have to build centralized water treatment facilities in order to comply with the new MCL. However, many small and mid-sized municipalities, as well as private users, cannot afford to build such facilities. Therefore, new technologies for removing arsenic need to be developed to meet the new MCL for arsenic.